The present invention relates to fluid jet print heads of the type used in ink jet printing and, more particularly, to a print head in which shutdown of the print head is facilitated, and in which the print head orifices are closed during periods in which the print head is not in operation.
Prior art fluid jet print heads typically include a manifold defining a fluid reservoir to which ink is supplied under pressure, and at least one orifice communicating with the reservoir. Ink from the reservoir flows through the orifice and forms a fluid filament. Mechanical disturbances are applied to the fluid filament, as for example by means of a piezoelectric transducer, to stimulate the fluid filament to break up into a jet drop stream. As drops are formed from the fluid filament, the drops are selectively charged and, thereafter, are deflected by an electrostatic field such that they are separated into print and catch trajectories. The drops in the print trajectories strike a print receiving medium, such as a paper web, while the drops in the catch trajectories are directed to a drop catcher, which ingests the drops and returns them to the ink supply system for reuse. One such prior art print head is shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis.
A significant problem encountered with fluid jet print heads occurs during shutdown of the print head. In order to terminate fluid flow through the orifice, typically the fluid pressure is rapidly reduced. This pressure reduction requires a finite time period, however, and at some point the fluid pressure within the print head is reduced to a degree that the jet drop streams tend to become unstable, both in trajectory and in drop size. As a result various components of the printer in which the print head is incorporated may be wetted to a degree that they can no longer function.
An additional problem exists with respect to such print heads during periods of nonuse. Either ink is removed completely from the print head during shutdown by a sequence of operations including flushing of the print head with a purge fluid and drying of the print head with air, or, alternatively, the ink is allowed to remain within the print head. The former shutdown procedure is relatively complicated, requiring additional valves and controls, as well as sources of pressurized air and purge fluid, while permitting fluid to remain in the print head raises the possibility of weeping of the print fluid through the orifices and wetting of various printer components.
U.S. Pat. No. 4,042,937, issued Aug. 16, 1975, to Perry et al, discloses an ink supply system in which sequencing of purging, start up, print operation, and shutdown of the printer are controlled by a pair of solenoid actuated valves connected in the inlet and outlet lines of the print head. The inlet valve is connected between a pump and the print head, while the outlet valve is connected between the print head and a supply tank which provides ink to the pump. At shutdown of the printer, the inlet valve is closed while the outlet valve is held open, thus creating a negative pressure in the head. The pump is then turned off.
Ink is maintained within the print head during the period of time in which the printer is shutdown. Perry et al suggests it may be desirable to purge the print head of air bubbles subsequent to shutdown to prevent drying of ink inside the print head. Nevertheless, drying of ink and precipitation of particulate contaminants may occur within the print head during protracted shutdown periods, with the result that the print head nozzles or orifices may become clogged. Additionally, there remains the possibility that ink may weep through the nozzles during periods of shutdown, producing undesirable wetting of various printer elements.
Other types of ink jet printers have included a provision for removing all of the ink from the print head reservoir during periods of printer shutdown in order to minimize clogging of the print head orifices and to reduce the possibility of weeping of ink through the orifices. This necessarily complicates start up and shutdown of the printer, however. U.S. Pat. No. 3,970,222, issued July 20, 1976, to Duffield, discloses such an ink jet printer.
U.S. Pat. No. 3,891,121, issued June 24, 1975, to Stoneburner, discloses a start up method in which the print head reservoir, initially dry, is prepressurized with air and a flushing liquid before supplying ink to the manifold. At shutdown of the printer, the flow of ink to the print head reservoir is replaced with a flow of flushing fluid. The flow of flushing fluid is then terminated and, simultaneously, an evacuation line leading to a low pressure source is opened, removing fluid from the manifold. The manifold is thereafter maintained in a dry condition until start up of the printer is subsequently initiated.
In order to avoid the difficulties encountered with print heads of the type from which ink is removed during shutdown periods, while at the same time eliminating the possibility of ink drying in the print head orifices and clogging the orifices, as may occur with printers of the type in which ink is maintained within the print head during shutdown periods, U.S. Pat. No. 3,839,721, issued Oct. 1, 1974, to Chen et al, discloses a printer arrangement having a liquid filled container which is movable with respect to the jet nozzles or orifices. The container, filled with water or water containing detergent, submerges the print head nozzles during shutdown periods and prevents drying of ink within the nozzles. In an alternative embodiment, the nozzles are submerged in a mist or vapor which prevents ink drying.
Such an arrangement adds significantly to the size, complexity, and costs of the printer. Additionally, contaminants in the ink within the print head may settle during periods of printer shutdown, causing the nozzles or orifices to become clogged. Similar arrangements are shown in U.S. Pat. No. 4,160,982, issued July 10, 1979, to Keur, and U.S. Pat. No. 4,144,537, issued Mar. 13, 1979, to Kimura et al. In these patents, devices external to the print head are provided to cap or cover the print head nozzle during periods of shutdown.
Other prior art devices have utilized valving arrangements within the print head for controlling fluid flow through an orifice. A marking device is disclosed in U.S. Pat. No. 3,730,133, issued May 1, 1973, to Cordiano et al, which includes a solenoid actuated needle valve within the fluid chamber. The needle valve is periodically opened during operation of the device to permit fluid flow through the orifice when desired. Opening the needle valve produces a slug of marking fluid which is directed at a print receiving surface. While such a needle valve arrangement permits the orifice to be closed during periods of nonuse, it is disadvantageous in that each orifice must necessarily have its own separate needle valve and associated controlling solenoid. As a consequence, it is not practical to utilize such a needle valve arrangement in a print head having a large number of closely spaced orifices.
Another arrangement for controlling fluid flow through an orifice internally of a marking head is shown in U.S. Pat. No. 4,109,282, issued Aug. 22, 1978, to Robertson et al. The Robertson device includes a writing head formed with a plurality of nibs. Each nib includes a duct formed with two passageways; one passageway supplies clear ink to the paper and the other supplies black ink to the paper. Strips of film material within the nib, having electrically conductive coatings, may be deflected so as to permit the application of either clear ink or black ink to the paper. The Robertson et al device is, however, not capable of terminating the flow of ink through the nib completely.
Finally, an arrangement for controlling the flow of ink through an orifice internally of a print head is shown in German published application No. 2905063, filed Feb. 10, 1979, published Aug. 14, 1980, and assigned to Olympia Werke A.G. The Olympia application discloses a piezoelectric pressure generator which presses a central diaphragm within the fluid reservoir of a print head toward the print head orifice. The diaphragm defines an opening which is covered by the transducer such that ink is forced from the orifice. On the return stroke of the transducer, the transducer moves away from the diaphragm opening, thus permitting the diaphragm to return ot its original position without drawing air into the reservoir through the orifice. The diaphragm is not utilized for terminating the flow of ink through the orifice, however.
Accordingly, it is seen that there is a need for a simple, reliable fluid jet print head in which ink flow through the print head orifice or orifices may be prevented during periods of print head shutdown and, further, in which the shutdown operation of the print head is facilitated.